(1) Field of the Invention
This invention relates to a method for the preparation of base/clay composites for the removal of SO.sub.x (sulfur dioxide and sulfur trioxide) from sulfur-containing gas streams, particularly flue gas streams from coal burning power plants. The base component in the composite is selected from the alkaline earth metal hydroxides and carbonates. The composite contains a second metal oxide or metal oxide precursor, preferably selected from transition metal ions, capable of promoting the oxidation of sulfur dioxide to sulfur trioxide.
(2) Prior Art
Coal represents the largest resource of fossil energy in the world, today. For example, it has been estimated that the known coal reserves in the U.S. alone could supply sufficient energy for domestic consumption for several hundred years. One of the major problems in using coal as an energy source is the presence of sulfur. In fossil-fuel-fired power plants, the sulfur content of the feed coal is oxidized during combustion to sulfur oxides (SO.sub.2 and SO.sub.3, commonly referred to as "SO.sub.x "), which are released through stacks to the atmosphere. Analyses of flue gas produced by power plants burning coal before desulfurization, show 0.5%-0.2% SO.sub.2 and about 0.005% SO.sub.3. One of the most serious environmental problem associated with such sulfur emissions is the generation of sulfuric acid, resulting in the so-called "acid rain".
Control of sulfur oxide emissions is mandated by the United States Environmental Protection Agency (EPA), and in 1970, the Clean Air Act Amendments were adopted by the U.S. government for this purpose. This legislation provided for enforcement, by the EPA, of SO.sub.x emissions limits for power plants constructed or modified after Aug. 17, 1971. This Act spurred extensive flue gas desulfurization (FGD) research and various studies are under way to develop methods for SO.sub.x removal from flue gas streams. As of January 1984, calcium based, wet, throwaway systems ( including lime, limestone, and alkaline-ash systems) accounted for 84 percent of existing and planned FGD capacity. The Clean Air Act was amended in 1977 and very recently in 1990, to require further control of SO.sub.x emissions. The Clean Air Act of 1990 requires, among other things, that coal-fired power plants cut sulfur dioxide emissions by half, or about 9 million tons annually, in the next decade. Increasing federal regulations and the high cost to construct and operate existing wet FGD units have encouraged continued research on new or modified flue gas cleanup processes.
Controlling the emissions of SO.sub.x from power plants is a world-wide problem and research into its control is a global effort. Formation of SO.sub.x in combustion processes can be reduced by modifying the burner design and combustion system, by changing the operating conditions and by using fuels with lower sulfur contents. The most popular and inexpensive method of reducing SO.sub.x emission is the addition of reactive dry sorbents with the fuel. Accordingly at present, SO.sub.x removal is most often accomplished by using lime (CaO), lime stone (CaCO.sub.3) or hydrated lime (Ca(OH).sub.2) due to cost effectiveness and available quantities. For example, in U.S. Pat. No. 4,731,233 by Thompson and Nuzio, describe the use of these calcium based sorbents to reduce SO.sub.x emissions from flue gas streams.
In typical coal-fired power plants the ground sorbent, for example lime or limestone, is added into boilers along with coal or sprayed into towers as a slurry to contact the flue gas stream. The SO.sub.2 reacts with calcium hydroxide to form a calcium sulfite slurry which is then partially oxidized with air to calcium sulfate. In this way the sulfur oxides are retained as harmless solid compounds which can be removed from the stack gas by electrostatic precipitation or other standard methods. Such a process is potentially attractive for retro-fitting existing power plants since no major structural alterations are required.
Although calcium based systems are the major source of SO.sub.x control they are not without problems. Agglomeration of particles can be a serious problem that results in a less than optimal conversion to CaSO.sub.x, (CaSO.sub.3 and CaSO.sub.4). The activity of the calcium species decreases as its particle size increases. Also CaSO.sub.x, occupies more volume than CaO, the common active species. Therefore, an increase in volume occurs as the reaction proceeds, which causes a loss in the original porous character of the CaO. This results in a blockage of SO.sub.x and O.sub.2 to the active CaO centers (Gullett, B. K. and Blom, J. A., React. Solids, 3 337 (1987); Gullett, B. K., Blom, J. A. and Cunningham, R. T., React. Solids, 6 263 (1988); Chang, E. Y. and Thodes, G., AIChE J., 30 450 (1984); Thibault, J. D., Steward, F. R. and Ruthyen, D. M., Can. J. Chem. Eng., 60 796 (1982)). Hence in the relatively short contact time available, only a small fraction of the sorbent reacts. In principle the problem of low utilization of the sorbents may be solved by reducing the particle size, but in practice, the particle size required for a reasonable level of utilization may be too small to achieve economically by conventional grinding or fragmentation methods.
Thermodynamic calculations indicate that the capture of sulfur trioxide with metal oxides is more favorable compared to sulfur dioxide. Several experimental results have suggested that catalytic oxidation of sulfur dioxide to sulfur trioxide can be beneficial for stack gas desulfurization. Kocheffe & Karman in Cand. J. Chem. Eng., 63, 971 to 977 (1985) has shown that the rate of reaction of SO.sub.3 with Ca, Mg and ZnO is greater than that of sulfur dioxide with the same oxides under identical conditions. Furthermore, inclusion of Fe.sub.2 O.sub.3 (as a SO.sub.2 oxidation catalyst) leads to more effective utilization of the lime. The addition of a small amount of Fe.sub.2 O.sub.3 gave both a more rapid initial uptake rate and a much higher final conversion of the lime (80-90%). In the absence of an oxidation catalyst the rate of SO.sub.2 absorption declined sharply at about 70% conversion.
Several methods have been used to develop reactive limestone, lime or hydrated lime as a precursor for the active CaO species or have used Ca(OH).sub.2 as the active species. Generally, the active species has been used as a bulk phase and not as a dispersed species (Chang, J. C. S. and Kaplan, N., Envir. Prog., 3 267 (1984); Gullett, B. K., Blom, J. A. and Cunningham, R. T., React. Solids, 6 263 (1988); Chang, E. Y. and Thodes, G., AIChE J., 30 450 (1984); Fuller El L. and Yoos, T. R., Lanqmuir, 3 753 (1987)). Recent work has concentrated on the addition of fly ash to Ca(OH).sub.2 to enhance its activity in SO.sub.x control (Jozewicz, W. and Rochelle, G. T., Envir. Prog, 5 219 (1986); Jozewicz, W., Chang, J. C. S., Sedman, C. B. and Brna, T. G., JAPCA, 38 796 (1988); Jozewicz, W., Chang, J. C. S., Sedman, C. B. and Brna, T. G., React. Solids, 6 243 (1988); Jozewicz, W., Chang, J. C. S., Sedman, C. B. and Brna, T. G., EPA/600/d-87/095, (NTIS PB87-175857/AS); Jozewicz, W., Chang, J. C. S., Sedman, C. B. and Brna, T. G., EPA/600/D-87/135, (NTIS PB87-182663). The fly ash is a siliceous material and formation of various calcium silicates can occur. Several diatomaceous earths, montmorillonite clays and kaolins have also been identified as containing reactive silica (Jozewicz, W., Chang, J. C. S., Sedman, C. B. and Brna, T. G., React. Solids, 6 243 (1988)).
U.S. Pat. No. 4,830,840 has recently, described a sorbent composition containing an alkaline earth metal, aluminum-containing spinel/clay compositions for SO.sub.x capture. This patent describes the use of kaolin clays as the matrix material.
U.S. Pat. No. 4,952,382 by van Broekhoven has recently disclosed a catalyst composition suitable for the refining of heavy sulfur- and metal-containing petroleum feeds. An "anionic clay" component present in the catalyst serves as the sorbent for removal of SO.sub.x from feed gas in fluidized catalytic cracking units. Anionic clays are primarily synthetic clays. Their natural abundance is very low. The clay layers in anionic clays are composed of non silicate materials and have found no cations in the clay gallery. As a result these clays do not undergo swelling in water.
(3) Objects
It is a principal object of the present invention to provide sorbent compositions suitable for diminishing SO.sub.x from flue gas streams particularly from coal-fired power plants. It is an object to provide sorbent compositions which give better SO.sub.x uptake in shorter time duration to overcome the low utilization of common oxide sorbents such as CaO and MgO due to mass transfer limitation and low reactivity of SO.sub.2. Further, it is an object of the present invention to provide sorbent composite materials which are inexpensive to produce. These and other objects will become increasingly apparent from the following description and the drawings.